Slope stability problems are receiving more attention in recent years. High intensity of rain is considered to be one of the many factors which trigger slope failures. Water infiltration affects physical and mechanical characteristics change of soil and then causes the decrease of soil strength. The increase of water content plays an important role in the slope stability though rainfall is not the only one factor which causes the slope instability. The objectives of this research were to investigate the effect of rain water infiltration of clayey silt parameter of soil such as water content degree of saturation void ratio cohesion internal friction angle and the hydraulic conductivity. The soil used in this research was sampled from Gunung Kawi-Malang in the East Java-Indonesia.
The initial water content was 5157 the undisturbed soil sample was then naturally and gradually air dried up to 12.5 25 37.5 50 62.5 75 82.5 and 100 of the initial water content The wetting process were carried out with the gradual increment water content of 25 wsat - Wi 50 wsat wi 75 wsat - Wi up to 100 wsat - Wi. The dimension of soil specimens were governed by the dimension of the used soil test apparatus. The Direct Shear test was performed on 2.4 cm in height and 5.6 cm in diameter of soil samples. The Infiltration Column tests were carried out on 10 cm in height and 6 cm in diameter of soil sample. The tube was equipped with resistively censors placed in the inner wall at 2 cm of distance. The 0.5 hours 1 hours 1.5 hours and 2 hours of rain water infiltration effects were simulated by applying 2 cm height of water on top of the soil specimens in both soil test apparatus. The height of water were kept constant during the rain infiltration simulation and the remained water was removed before the Direct Shear test The Infiltration Column test were carried out to measure the rate of water infiltration on both un saturated and nearly saturated soil samples. The hydraulic conductivity of soil sample was then calculated using the Darcy formula The soil suction was measured using the no. 42 Whaltman filter paper. The falling head test was used to measured the saturated hydraulic conductivity.
The results show that during the wetting process from initial to saturated condition the degree of saturation was increased 28.32 the negative pore pressure was decrease 9462 the cohesion was decrease 8288 and the internal friction angle was decrease 4918. Upon drying from initial to drying condition the degree of saturation decrease 7104 the negative pore pressure increase 43935 the cohesion increase 15836.
It can be obviously seen that the water infiltration affect the physical and mechanical characteristics of the soil specimens. The increase of water content affected to the decrease of negative pore water pressure the cohesion and the friction angle From the initial of ws 51.57 -Ui - 30245 kPa to Ihe 100 degree of saturation resulted in the Wf 70.69 -Uf 1628 kPa. It can be obviously seen
from the Infiltration Column test that the hydraulic conductivity of soil varied with the initial water content of the soil tested. The decrease of water content affected to the decrease of the hydraulic conductivity of soil From the initial of w 51.57 km 1388 x 107 ms to the 2189 degree of saturation resulted in the wf 8.83 k 4933 x 108 ms. To better understand the effect of the rain water infiltration to the slope stability of soil at the natural water content the numerical analysis was performed using the soil properties measured in the laboratory test. Two slope angles 1H IV and 2H IV were analyzed in combination with three slope heights 10m 15m and 20m. The rain water infiltration was simulated using the SEEPW numerical program. The safety factor was then calculated using the SLOPEW. The results show that for 2 hours duration the 10 m height of slope remained stable the 15 m height af slope remained stable for 1H IV slope angle but unstable for 2H 1V while the 20 m height of slope become unstable for both of the slope angles. From the results above it is thought to be due that the slope heights slope angles and the rainfall duration are the factors most significantly affected the slope stability of the soil tested.

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